In a sampling format of CD (Compact Disc), a sampling frequency is 44.1 kHz. Therefore, in a music signal recorded on the CD, a high frequency range over an audible frequency range of a human (20 to 200 kHz) is cut.
Further, in compressed music signals such as MP3 or WMA, a high frequency range to which a human hardly listen is cut for reducing its file size. Therefore, there is a problem that sound recorded on the CD is deteriorated by compression of music signals. Accordingly, a harmonic sound generator is proposed for restoring the high frequency range by generating harmonic sound from the music signals.
For example, in Patent Document 1, a harmonic sound adding device for separately generating even-order harmonic sound and odd-order harmonic sound and for controlling a balance between the even-order and odd-order harmonic sound is described. The even-order harmonic sound is harmonic sound including frequency components of the music sound multiplied by even numbers, namely, 2, 4, 6, 8 . . . 2n (where n is an integer). On the other hand, the odd-order harmonic sound is harmonic sound including frequency components of the music sound multiplied by odd numbers, namely, 3, 5, 7, 9 . . . 2(n+1) (where n is an integer). Further, in Patent Document 2, an acoustic signal processor for generating harmonic sound of a music sound multiplied by integer numbers using a full-wave rectifying circuit is described. Every harmonic sound generator generates harmonic sound of the frequencies of the music signal multiplied by integer numbers.    [Patent Document 1] Japanese Published Patent Application No. H08-95567    [Patent Document 2] Japanese Published Patent Application No. 2004-101797